Tank-venting system for a motor vehicle as well as a method and an arrangement for checking the operability thereof

ABSTRACT

A method for determining the operability of a tank-venting system on a motor vehicle subjects the signals for the volume flow through the tank-venting valve and the signals for the pressure difference between the tank interior and the ambient to a cross-covariance analysis. The above-mentioned signals are formed by a high pass in advance of forming the cross-covariance function and the maximum or the mean value of the cross-covariance function is formed with respect to the product of the two input variables. A variance measure is formed for the signal of the volume flow through the tank-venting valve and a transfer factor is computed from the variance measure and the mean value or maximum. The tank-venting system is deemed to be operational when the transfer range lies in a pregiven region. An advantage of the method is seen in the independence of the tank-pressure changes which are not caused by volume-flow changes through the tank-venting valve. Here, changes are especially of concern which are caused by the sudden generation of vapor in the tank such as caused by sloshing fuel.

FIELD OF THE INVENTION

The following relates to a tank-venting system for a motor vehicle aswell as a method and an arrangement for checking the operability of sucha system.

BACKGROUND OF THE INVENTION

A tank-venting system typically has the following components: a tankhaving a tank closure device; an adsorption filter with a venting line;a tank-venting valve; a tank connecting line between the tank and theadsorption filter; and, a valve line between-the adsorption filter andthe tank-venting valve.

The tank-venting valve is connected to the intake pipe of an internalcombustion engine so that fuel vapors are drawn out of the tank-ventingsystem with the aid of the underpressure in the intake pipe. During thisprocess, the adsorption filter is regenerated with the aid of air whichis drawn in via the venting line.

There is the danger that tank-venting systems develop leaks or thatobstructions arise. Such systems therefore have to be repeatedly checkedas to operability during the operation of a motor vehicle.

The most important method for checking the operability of a motorvehicle tank-venting system is based on a proposal of the CaliforniaEnvironmental Authority CARB. According to this method, when opening thetank-venting valve, a check is made as to whether a lambda controllerhas to carry out a correction of its control output. This is always thecase when air with fuel vapor is evacuated by suction out of thetank-venting system. However, it is also the case that the adsorptionfilter is completely regenerated and that the fuel in the tank iscompletely evaporated. When the tank-venting valve is opened, no fuel issupplied in addition to that which is supplied to the injection valvesof the internal combustion engine according to the control output of thelambda control. In such a case, in which therefore no fuel is suppliedby the tank-venting system, that is, the lambda controller does not haveto carry out a correction, it is unclear whether the tank-venting systemhas developed a leak or whether no fuel is being supplied because of thementioned reasons. In order to be able to decide this question,according to the known method, an evaluation of the signal from thelambda controller only occurs when a fuel temperature sensor indicatesthat a predetermined minimum fuel temperature is exceeded and a tankfill-level sensor indicates that the vehicle tank has been filled. It isassumed that fuel vapor would then have to be present in the system inany event which vapor is drawn in when the tank-venting valve is openedand which then leads to a correction of the lambda controller. However,with this method incorrect decisions repeatedly occur if there is infact evaporated fuel in the tank, refilling occurs with the same kind offuel and the adsorption filter is largely regenerated.

A tank-venting system is described in U.S. Pat. No. 5,193,512 whichincludes a controllable shut-off valve in the venting line of theadsorption filter. This shut-off valve makes it possible to carry out amethod wherein the shut-off valve is closed, the tank-venting valve isopened and then a check is made as to whether an underpressure isbuilding up in the tank. If this is the case, it is concluded that thesystem is operable.

Erroneous decisions can be made in the above-mentioned method when thefuel vaporizes with great intensity. Furthermore, it is necessary tocarry out a special test cycle with the shut-off valve closed whereinthe adsorption filter cannot be regenerated.

Accordingly, the problem is presented to provide an especially reliablemethod for checking the operability of the tank-venting system for amotor vehicle as well as to provide an arrangement for carrying out sucha method and a tank-venting system having an operability which can bechecked both reliably and to a great extent.

SUMMARY OF THE INVENTION

The method of the invention for determining the operability of atank-venting system of the kind described above is characterized by thefollowing steps:

forming a first input variable for a cross-covariance analysis by ahigh-pass processing of the signal for the volume flow through thetank-Venting valve;

forming the second input variable for the cross-covariance analysis by ahigh-pass processing of the signal for the tank-pressure difference,that is, the difference between the pressure in the tank and the ambientpressure;

determining the maximum or mean value of the cross-covariance functionconcerning the product of the two input variables;

forming a measure for the variance of the first input variable;

computing a transfer factor by dividing the maximum or the mean value ofthe cross-covariance function by the above-mentioned measure for thevariance of the first input variable; and,

checking whether the transfer factor lies in a pregiven value range and,if so, determining the tank-venting apparatus as being operable,otherwise, as being inoperable.

The arrangement according to the invention includes a unit for eachabove-mentioned method step which unit is so configured that it carriesout the particular method step.

The tank-venting system according to the invention includes theabove-delineated features for a known tank-venting system and ischaracterized in that:

a venting throttle is provided on the adsorption filter;

the venting line of the adsorption filter leads up to the tank-closureunit and is mounted thereon so that the venting line is closed when thetank-closure unit is closed; and,

an arrangement according to an embodiment of the invention is provided.

The method according to the invention utilizes the realization thatchanges in the volume flow through the tank-venting valve take placealmost continuously during operation of a tank-venting system during thetank-venting phases. Tank pressure changes must correlate with thesevolume flows. On the other hand, tank-pressure changes can be caused byother effects, for example, by sloshing of the tank content when drivingthrough a curve and by suddenly generated vapors caused thereby;however, these pressure changes do not correlate to the volume flowthrough the tank-venting valve. Accordingly, it must be determinedwhether tank-pressure changes are caused by volume-flow changes (whichis an indicia for the operability of the system) by means of across-correlation analysis or, still better, by means of across-covariance analysis as in the invention.

To obtain input variables for the cross-covariance analysis, the signalsfor the volume flow and the tank-pressure difference, that is, thedifference between the pressure in the tank and the ambient pressure,are each subjected to high-pass processing. This preferably takes placein that the mean value of the particular signal is formed and this meanvalue is subtracted from the particular instantaneous signal value. Inthis way, the input variables fluctuate about the particular mean valueas is required for the input variables of a cross-covariance analysis.

The volume flow through the tank-venting valve can be measured directly;however, it is more advantageous to determine the volume flow with theaid of the following: the pressure difference present on thetank-venting valve, a pressure-difference volume-flow characteristic andthe pulse-duty factor of the drive of the tank-venting valve. Thepressure difference can either be measured or the pressure can bedetermined as the difference between the ambient pressure and the intakepressure which is, in turn, more advantageous. The intake pressure caneither be measured or can preferably be determined from a load signal.The ambient pressure can be assumed, as a good approximation, to beconstant; however, the ambient pressure can also be measured or bedetermined with the aid of substitute variables.

A product is formed from a pressure difference and a volume-flowdifference when forming the cross-covariance and this product isintegrated. Preferably, the integration is replaced by a low-passfiltering. The obtained value can be divided by the variance of thevolume-flow difference, that is, by the integrated square or preferablyby the low-pass filtered square of the volume-flow difference. If thisis done, then as a transfer factor, a variable is obtained whichexpresses the extent to which the pressure in the tank changes with achange of the volume flow.

The value of the cross-covariance function relating to the product ofthe two input variables then becomes a maximum when values of both inputvariables are multiplied by that time-dependent shift which correspondsto the phase shift between the two signals. Thus, the particularinstantaneous tank differential-pressure difference must be multipliedby a previous volume-flow difference. Previous values of this kind mustbe stored over a time range which lies between the minimum possible andthe maximum possible phase shift.

If a fill-level sensor is provided in the tank, then the phase shift canbe determined with the aid of a fill-level phase-shift characteristic.That volume-flow difference is selected from the stored volume-flowdifferences for multiplication with the instantaneous tankdifferential-pressure difference which just has the determined phaseshift. If, in contrast, no fill-level sensor is provided, then it isadvantageous to multiply all stored volume-flow differences by theparticular instantaneous tank differential-pressure difference and tolow-pass filter all products. The values obtained in this manner arerepresentative of the cross-covariance function. The maximum value caneasily be selected from these values.

It is apparent that the individual values of the cross-covariancefunction are not only dependent from the covariance between the twoinput variables but also from the absolute value of the variables. Theabsolute value of the tank differential-pressure difference is dependenton whether the tank-venting system leaks or is blocked. The entirecross-covariance function is affected thereby. This has the consequencethat not only the maximum of the cross-covariance function can be usedto form the transfer factor but also the mean value of thecross-covariance function. However, more precise results are provided bythe maximum according to the theory of the cross-covariance analysis.

The result of he cross-covariance analysis becomes more precise, thebroader in range the time-dependent changes of the volume flow throughthe tank-venting valve are. For cases where the volume-flow signaloccurring during normal operation does not have an adequate frequencyband width, it is advantageous to change the pulse-duty factor of thetank-venting valve arbitrarily. This can take place continuously or onlythen when the absolute mean value of the volume-flow differences dropsbelow a threshold value which indicates that hardly any more changesoccur with which changes in the tank differential pressure could becorrelated. However, it is only purposeful to undertake these changes ofthe pulse-duty factor when the intake-pipe pressure is so low that thechanges of the pulse-duty factor actually lead to volume-flow changes.Whether this is the case, can be decided either directly with the aid ofthe intake-pipe pressure or with the aid of a lower threshold value forthe absolute mean value of the volume-flow difference.

It must be guaranteed that changes of the volume flow continue throughthe tank-venting valve into the tank as well as possible so that themethod and the arrangement of the invention for determining theoperability of a tank-venting system operate properly. This is the case,especially when the tank-venting valve is connected directly to the tankand the adsorption filter is only connected via a connecting line to theabove-mentioned line. If in contrast, the tank-connecting line isintroduced relatively far into the adsorption filter and if theadsorption filter is connected at its suction end separately to thetank-venting valve, it is possible that pressure changes on the suctionend (caused by volume-flow changes) hardly affect the tank pressure.However, the measuring effect can be raised also in this case when anarrowly throttled venting line is used.

If a throttled venting line is used, then the problem arises, duringtanking, in motor vehicles having internal fuel recovery that, duringtanking, adequate air cannot escape through the venting line. In thiscase, it is advantageous if the adsorption filter has a venting line inaddition to the venting throttle which can be opened during tanking butis closed during normal operation of the tank-venting system. In anespecially advantageous manner, this venting line leads to thetank-closure unit and this closure unit is so configured that it closesthe venting line in the closed state. In this way, it is automaticallyguaranteed that the venting line is opened when tanking. If the closureis no longer actuated after tanking, this has the consequence that theventing line remains open which, in turn, has the consequence thathardly any underpressure develops in the tank even for large volumeflows through the tank-venting valve so that the cross-covarianceanalysis supplies very low transfer factors which indicate that thesystem leaks.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a tank-venting system equippedwith an arrangement for determining the operability of the System;

FIG. 2 is a schematic representation of a tank-closure unit with thetank closure open and on which the venting line for the adsorptionfilter shown in FIG. 1 ends;

FIG. 3 is a block function diagram for the arrangement included in FIG.1 for determining the operability of the tank-venting system;

FIG. 4 is a block function diagram for explaining how the inputvariables are obtained for the cross-covariance analysis undertaken inthe function sequence according to FIG. 3;

FIG. 5 explains a block function diagram as to how the variance of oneof the input variables is determined;

FIG. 6 is a block function diagram for explaining how the maximum valueor mean value of a cross-covariance function is determined in thesequence according to FIG. 3;

FIG. 7 is a block function diagram corresponding to that of FIG. 6 butfor another variant to determine a mean value in connection with across-covariance analysis; and,

FIG. 8 is a block function diagram corresponding to that of FIG. 6 butfor another variant for determining the maximum value of thecross-covariance function.

It is noted that all block function diagrams can be understood either asa block diagram for an arrangement or as a sequence diagram for amethod.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The illustration of FIG. 1 includes a tank-venting system having a tank10 with a tank-closure unit 11, an adsorption filter 12 connected to thetank and having a venting line 13 and a venting throttle 14 as well as aventing line 15 to the intake pipe 16 of an internal combustion engine17. A tank-venting valve 18 is inserted in the tank-venting line 15. Ahot-wire air-mass sensor 19 is disposed in the intake pipe and emits anair-mass signal LM. A speed sensor 20 on the engine 17 determines thespeed (n) thereof.

The tank-venting valve 18 is driven by a valve-control unit 21 having apulse-duty factor which is dependent upon the time (t) and the load ofthe engine which is determined from the values LM and (n). The timedependence comprises that tank venting is permitted only duringso-called tank-venting phases. Continuous tank venting is not permittedsince the lambda control is affected by the tank venting. However, thelambda control must be adapted without the unknown influence of the tankventing. Typically, the adaptation phases and the tank-venting phaseseach take several minutes.

A test unit 22 is provided for checking the operability of thetank-venting system. The test unit 22 emits an evaluation signal BSwhich indicates the result of the check. The test unit 22 receives adifferential-pressure signal dP from a differential-pressure sensor 23mounted on the tank 10. The pressure sensor 23 measures the differencebetween the pressure in the tank and the ambient pressure. Furthermore,the test unit 22 receives a load signal for determining the pressure inthe intake pipe 16. The load signal is here, in turn, represented by thevalues of the air mass LM and the speed (n) as well as selectively afill-level signal FST from a fill-level sensor 24 in the tank.

From FIGS. 1 and 2 it is apparent that the venting line 13 is closed bythe tank closure 25 as long as the cover itself is closed (FIG. 1). Incontrast, this line is opened when the closure is opened (FIG. 2). Thepurpose of this arrangement is explained further below.

The basic function of the test unit 22 will now be described on thebasis of FIG. 3.

The signal VS₋₋ TEV(t) for the volume flow through the tank-ventingvalve 18 is subjected to high-pass processing in a first input-variabledetermination unit 26. The differential-pressure signal dP(t) from thedifferential-pressure sensor 23 is subjected to a correspondinghigh-pass processing in a second input variable determination unit 27.In this way, the input variables ΔVS₋₋ TEV(t) and ΔdP(t) are obtainedfor a cross-covariance analysis which is carried out in across-covariance unit 28. The first input variable is further suppliedto a variance-determination unit 29 as well as to a unit 30 fordetermining the mean of the absolute values of the first input variable.In a quotient former 31, the variance of the first input variable isdivided by the maximum or the mean value of the cross-covariancefunction relating to the two input variables in the manner in which thismean value is emitted by the cross-covariance unit 28. The quotientobtained is compared to a lower and an upper threshold value for thequotient in a decision unit 32. If there is a drop below the lowerthreshold value or if the upper threshold value is exceeded, then thevalue "1" is emitted as the decision signal BS which indicates that thetank-venting system is not operational. Otherwise, the value "0" isemitted.

The mentioned high-pass processing of the signals VS₋₋ TEV(t) and dP(t)is now explained with respect to FIG. 4. There, only the firstinput-variable determining unit 26 is shown which operates as a highpass; however, the unit 27 for determining the second input variableoperates in a corresponding manner. The volume-flow signal VS₋₋ TEV(t)is supplied to the first determination unit 26. This signal is averagedin a low pass and the volume-flow mean value is subtracted from theactual volume flow at an addition point whereby the volume-flowdifference ΔVS₋₋ TEV(t) is obtained as the first input variable.

The volume flow through the tank-venting valve 18 is determined in theembodiment as follows. The intake-pipe pressure pS is determined independence upon the load of the engine 17 from a load/intake-pipepressure characteristic field 33. In the illustration of FIG. 4, valuesof the air-mass signal LM and the speed (n) are used as loadinformation. The volume flow through the tank-venting valve isdetermined from the intake-pipe pressure with the aid of apressure-difference/volume-flow characteristic 34 as the volume flowapplies for a completely open valve and constant pressure on the end ofthe tank-venting valve facing toward the tank. The last condition isrelatively well satisfied since the pressure on the above-mentioned endalways corresponds essentially to the air pressure which is assumed inthe embodiment to be constant. Finally, the volume flow determined inthis manner is modified in a multiplication point by the pulse-dutyfactor τ of the tank-venting valve in order to obtain in this way theactual volume flow VS₋₋ TEV(t). This sequence can be simplified ifeither the intake-pipe pressure is measured or even if the volume flowthrough the tank-venting valve is determined directly by a through-flowsensor.

FIG. 5 shows the typical operation of a variance determination. Theinput variable, which is here the volume-flow difference ΔVS₋₋ TEV(t),is quadrupled and then integrated. In a preferred embodiment, a low-passfiltering through a low pass TP is undertaken in lieu of integration.

The operation shown in FIG. 6 is that preferred as the operation of thecross-covariance unit 28. Alternatives are explained with respect toFIGS. 7 and 8.

According to FIG. 6, values for the volume-flow difference ΔVS₋₋ TEV(t)are stored sequentially in a FIFO-memory 35 at pregiven time intervalsT; that is, so many values are stored that the largest time shift N·Tcorresponds to the maximum possible phase shift between a change of thevolume flow and a change of the tank differential pressure. The oldestvalue drops out of the memory with each newly received value. The storedvalues except for the most current one are all multiplied in multiplierpoints 36.1 to 36.n by the current tank differential-pressure differenceΔdP(t) as it is emitted from the first input-variable determination unit27. The current volume-flow difference is not multiplied since, in theembodiment, the minimum possible mentioned phase difference is not zero;instead, it corresponds at least to the time span with which thesequentially following difference values are received in the memory.Each of the individual products is low-pass filtered in a downstreamlow-pass filter (TP) 37.1 to 37.N (ideally, an integration over theentire measuring time span should take place), whereby the individualvalues of a cross-covariance function are formed. From these values, themaximum value or the mean value is determined which takes place in aunit 38.

The time-dependent total sequence will be described before variations ofthe preferred embodiment are explained.

Measured values are formed every 30 msec for the output variables, thatis, the volume flow VS₋₋ TEV(t) and the tank differential pressuredP(t). The described low-pass filtering takes place by means of asliding mean-value formation pursuant to which the leading mean value isweighted by approximately 99%, preferably more and the new value isweighted with the remainder to 100%. This corresponds to a time constantof approximately 5 seconds. The volume-flow difference ΔVS₋₋ TEV(t)formed in this manner is read into the FIFO-memory 35 every 500 msec.The FIFO-memory 35 holds a total of 30 values which means that theoldest value still held is 15 seconds old. This corresponds to a phaseshift for the tank-venting system of the embodiment for a tank which isalmost empty. For a full tank, the phase shift amounts to onlyapproximately 500 msec. For all phase shifts, the cross-covariance tothe tank differential-pressure difference ΔdP(t) is computed with theaid of the multiplication points 36.1 to 36.n and then low-passfiltered. This filtering takes place in the embodiment with a timeconstant of 6 minutes which means that for a computation every 500 msec,the value which is new each time is only considered with approximately1% in proportion to the old mean value. The low-pass filtered valuewhich is the greatest is dependent upon the fill level of the tank atthe particular time.

The sliding mean-value formation in the low pass takes place in thevariance-determination unit 29 with a time constant of likewise 6minutes. If the value determined in this manner is divided by the outputvariable of the cross-covariance unit 28, then a transfer factor resultswhich indicates how large the tank differential-pressure changes must befor pregiven volume-flow changes. The transfer factor for a properlyoperating tank-venting system can be determined by experiments. A bottomthreshold value lying somewhat lower and an upper threshold value lyingsomewhat higher are determined with the aid of the transfer factordetermined in this manner. If there is a drop below the lower thresholdvalue, this indicates that the tank,venting system is not operationalbecause of a leak. If the upper threshold value is exceeded, thisindicates inoperability because of a blockage of the adsorption filter.In this case, the pressure changes in the tank for volume-flow changesthrough the tank-venting valve are greater than they should actually be.

Cross-covariances can only then be purposefully formed when volume-flowchanges are present which can cause pressure changes in the tank. If thevolume-flow changes are too small, it is therefore advantageous to delaythe evaluation described below. For this purpose, the output signal ofthe absolute value averaging 30 is supplied to a first threshold checkunit 39.u which checks whether the mean value of the absolute values ofthe volume-flow changes has dropped below a lower threshold value. Ifthis is the case, then a signal is emitted to the variance-determinationunit 29 and the cross-correlation unit 28 to maintain the particularlast value.

In the last case, the method for determining the operability of thetank-venting system must be delayed. In order to possibly avoid thislast case, the output signal of the absolute mean value averaging 30 issupplied to a second threshold check unit 39.o. The unit 39.o emits asignal ZFS for generating an arbitrary pulse-duty factor τ to thevalve-control unit 21 as soon as there is a drop below theabove-mentioned threshold value. However, the valve-control unit 21generates this arbitrary pulse-duty factor only when this unitdetermines with the aid of the load signal that this pulse-duty factorhas a purpose because of an adequately low intake-pipe pressure and ifthe unit furthermore determines from the operating-condition data of theengine 17 that the engine runs in an operating range wherein continuouschanges of the pulse-duty factor do not constitute a disturbance.

An arbitrary pulse-duty factor can basically always be adjusted as longas an operating range of the engine is present wherein the changes ofthe pulse-duty factor do not constitute a disturbance. This is done inorder not only to obtain a volume-flow change signal having the greatestpossible band width for the above-mentioned conditions but also in orderto have one such signal always present. However, care must be taken thatthe time-dependent mean value of the pulse-duty factor correspondsapproximately to that pulse-duty factor which would be adjusted forconventional operation of the valve-control unit in the particularoperating state.

A simplification of the embodiment of FIG. 6 is explained with referenceto FIG. 7. According to FIG. 7, a mean-value formation and then alow-pass filtering of this mean value takes place directly after theformation of the products in the multiplication points 36.1 to 36.n.This is in lieu of first low-pass filtering the product and thendetermining the mean value. In this way, all low-pass filters except forone are made unnecessary. The output signal obtained in this manner fromthe cross-covariance unit 28.1 according to FIG. 7 is however lessreliable than that from the cross-covariance unit 28 according to FIG. 6since the correct determination of the cross-covariance function firstrequires the low-pass filtering.

The cross-covariance unit 28.2 according to FIG. 8 operates with thesame reliability as the cross-covariance unit 28 according to FIG. 6.However, the cross-covariance unit 28.2 operates with less computationcomplexity but requires a level sensor 24. The-cross-covariance unit28.2 also has the FIFO-memory 35 described with respect to FIG. 6.However, only the value for multiplication by the tankdifferential-pressure difference ΔdP(t) is read out from the FIFO-memory35 and this value has precisely the actual phase shift to theabove-mentioned pressure difference. This phase shift is obtained withthe aid of a fill-level/phase-shift characteristic 40 which emits thecorresponding phase shift between volume-flow change and tank-pressurechange for every filling level of the tank. The read out value isfiltered in a single low pass 37 after the above-mentionedmultiplication. Accordingly, the multiplier points except for one, thelow-pass filters except for one and the determination unit 38 for themaximum value of the cross-correlation function are unnecessary.

We claim:
 1. A method for determining operability of a tank-ventingsystem on a motor vehicle having an internal combustion engine, thetank-venting system having an adsorption filter which is connected via atank-connecting line to a tank and via a valve line to an intake pipe ofthe engine with a tank-venting valve connected therebetween throughwhich a volume flow passes, the method comprising the steps of:providinga cross-covariance unit defining a cross-covariance function forcarrying out a cross-covariance analysis therein; measuring the volumeflow through said tank-venting valve and providing a signal indicativeof said volume flow; forming a first input variable for saidcross-covariance analysis by high-pass processing said signal for thevolume flow through the tank-venting valve; measuring the differencepressure between the pressure in the tank and the ambient pressure andproviding a signal indicative of said difference pressure; forming asecond input variable for the cross-covariance analysis by high-passprocessing said signal for said difference pressure between the pressurein the tank and the ambient pressure; determining one of the maximumvalue and the mean value of the cross-covariance function with respectto the product of the two input variables; providing avariance-determination unit; applying said first input variable to saidvariance-determination unit to obtain a measure of the variance of thefirst input variable; computing a transfer factor by dividing said oneof the maximum value and said mean value of the cross-covariancefunction by said measure for the variance of the first input variable;and, checking whether the transfer factor lies in a pregiven value rangeand if this is the case, determining the tank-venting system as beingoperational, otherwise, as being non-operational.
 2. The method of claim1, wherein the maximum of the cross-covariance function is determined bythe further method steps of:sequentially storing a plurality of valuesof said first input variable at pregiven time intervals with a first oneof the stored values corresponding to the smallest shift in phasebetween a change of said volume flow and a change in said differencepressure to a last one of said stored values corresponding to themaximum possible shift in phase between a change of said volume flow anda change of said difference pressure; multiplying each of the storedvalues except for the most recent one by successive values of saidsecond input variable to form a plurality of individual products;computing said cross-covariance for each of said shifts in phase; and,selecting the cross-variance function having the largest positive value.3. The method of claim 1, wherein the maximum of the cross-covariancefunction is determined by the further method steps of:determining thelevel of the tank; determining a phase shift to be expected between thefirst and the second input variables; multiplying the actual value ofthe second input variable by the value of the first input variable timedelayed by the determined phase shift; and, averaging the multiplicationresult.
 4. The method of claim 1, wherein the mean value of thecross-covariance function is determined by the further method stepsof:continuously computing a plurality of products from the actual valueof the second input variable and a time-displaced value of the firstinput variable with the values of the first input variable extendingfrom that value having the smallest possible phase shift with respect tothe first input variable up to the value having the largest possiblephase shift; computing the cross-covariance factor for each phase shiftconsidered; and, computing the mean value of the cross-covariancefactors.
 5. The method of claim 1, wherein the mean value of thecross-covariance function is determined by the further method stepsof:continuously computing a plurality of products from the actual valueof the second input variable and a time-shifted value of the first inputvariable with the values of the first input variable extending from thatvalue having the smallest possible phase shift with respect to the firstinput variable up to that value having the largest possible phase shift;summing all products; and, averaging the product sum.
 6. The method ofclaim 1, wherein the high-pass processing for the volume flow takesplace by the further method steps of:applying said signal indicative ofsaid volume flow to a high-pass filter to form a volume-flow mean value;and, computing the volume-flow difference between the actual volume flowand the volume-flow mean value.
 7. The method of claim 6, wherein thehigh-pass processing for the tank differential pressure takes place bythe further method steps of:applying said signal indicative of saiddifference pressure to form a mean value for the tank differentialpressure; and, computing the tank differential-pressure differencebetween actual tank differential pressure and tank differential pressuremean value.
 8. The method of claim 7, wherein the tank-venting valve isdriven by a valve drive unit at a pulse-duty factor; and wherein themethod comprises the further steps of:applying said first input variableto an absolute value averaging unit to obtain an absolute value signal;applying said absolute value signal to a threshold unit to obtain arandom function signal; and, applying said random function signal tosaid valve drive unit to adjust the pulse-duty factor of thetank-venting valve.
 9. The method of claim 8, wherein the pulse-dutyfactor of the tank-venting valve is only adjusted by said randomfunction signal after satisfying the following conditions:theintake-pipe pressure lies below a pressure threshold; the value-flowmean value falls below an upper mean-value threshold; and, the engineoperates in an operating region wherein continuous changes of thepulse-duty factor do not cause electrical disturbance.
 10. The method ofclaim 9, wherein all method sequences are delayed in such time frameswherein the value-flow mean value drops below a lower mean-valuethreshold.
 11. An arrangement for determining operability of atank-venting system on a motor vehicle having an internal combustionengine, the tank-venting system having an adsorption filter which isconnected via a tank-connecting line to a tank and via a valve line toan intake pipe of the engine with a tank-venting valve being connectedtherebetween through which a volume flow passes, the arrangementcomprising:means for measuring said volume flow through saidtank-venting valve and providing a signal indicative of said volumeflow; first input-value determination means for high-pass processing thesignal for the volume flow through the tank-venting valve fordetermining a first input variable; means for measuring the differencepressure between the pressure in the tank and the ambient pressure andproviding a signal indicative of said difference pressure; secondinput-variable determination means for high-pass processing the signalfor said difference pressure between the pressure in the tank and theambient pressure for determining a second input variable;cross-covariance means defining a cross-variance function fordetermining one of the maximum value and the mean value of thecross-covariance function relating to the product of said two inputvariables; variance-determination means for forming a measure for thevariance of the first input variable; quotient former means forcomputing a transfer factor by dividing said one of the maximum valueand the mean value of the cross-covariance function by said measure forthe variance of the first input variable; and, decision means fordetermining whether the transfer factor lies in a pregiven value rangeand for determining the tank-venting system as being operational if thisis the case, otherwise determining the system as being non-operational.12. A combination of a tank-venting system in a motor vehicle equippedwith an internal combustion engine having an intake pipe and anarrangement for determining operability of said tank-venting system, thecombination comprising:a tank-venting system including: a tank having atank-closure unit; an adsorption filter having a venting line; atank-venting valve through which a volume flow passes; a tank-connectingline between said tank and said adsorption filter; a first valve lineinterconnecting said adsorption filter and said tank-venting valve; asecond valve line interconnecting said tank-venting valve and saidintake pipe; a venting throttle arranged on said adsorption filter; aventing line interconnecting said adsorption filter and saidtank-closure unit; and, said venting line being mounted on saidtank-closure unit so that the venting line is closed in the closed stateof the tank-closure unit; and, said arrangement including: means formeasuring said volume flow through said tank-venting valve and providinga signal indicative of said volume flow; first input-value determinationmeans for high-pass processing the signal for the volume flow throughthe tank-venting valve for determining a first input variable; means formeasuring the difference pressure between the pressure in the tank andthe ambient pressure and providing a signal indicative of saiddifference pressure;second input-variable determination means forhigh-pass processing the signal for said difference pressure between thepressure in the tank and the ambient pressure and for determining asecond input variable; cross-covariance means defining across-covariance function for determining one of the maximum value andthe mean value of the cross-covariance function relating to the productof said two input variables; variance-determination means for forming ameasure for the variance of the first input variable; quotient formermeans for computing a transfer factor by dividing said one of themaximum value and the mean value of the cross-covariance function bysaid measure for the variance of the first input variable; and, decisionmeans for determining whether the transfer factor lies in a pregivenvalue range and for determining the tank-venting system as beingoperational if this is the case, otherwise determining the system asbeing non-operational.